ZyXEL Communications P-2612HW-F1 - Especificaciones descargar pdf (Pagina 19)

NAT Introduction

 What is NAT?

NAT (Network Address Translation-NAT RFC 1631) is the translation of an

Internet Protocol address used within one network to a different IP address known

within another network. One network is designated as the inside network and the

other is the outside. Typically, a company maps its local inside network addresses to

one or more global outside IP addresses and "unmaps" the global IP addresses on

the incoming packets back into local IP addresses. The IP addresses for NAT can be

either fixed or dynamically assigned by the ISP. In addition, you can designate servers,

e.g., a Web server and a Telnet server, on your local network and make them

accessible to the outside world. If you do not define any servers, the NAT offers the

additional benefit of firewall protection. In such case, all incoming connections to

your network will be filtered out by the CPE, thus preventing intruders from probing

your network.

For more information on IP address translation, please refer to RFC 1631, The IP

Network Address Translator (NAT).

 How does NAT work?

According to the following figure, we define the local IP addresses as the Internal

Local Addresses (ILA) and the global IP addresses as the Inside Global Addresses

(IGA). The term 'inside' refers to the set of networks that are subject to translation.

The NAT operates by mapping the ILA to the IGA required for communication with

hosts on other networks. It replaces the original IP source address (and TCP or UDP

source port numbers) and then forwards each packet to the Internet ISP, thus

making them appear as if they came from the NAT system itself (e.g., the CPE router).

The CPE keeps track of the original addresses and port numbers, so the incoming

reply packets can have their original values restored.

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ZyXEL Communications P-2612HW-F1 - Especificaciones Pagina 19